中文摘要：
环丙烯因其较大的环张力而表现出较好的反应活性,环丙烯取代物的Diels-Alder反应是生物正交化学以及有机化学领域的一个研究热点。先前的研究表明,2-硼杂环[1.1.0]1(3)丁烯(2BB)是一类结构上类似于环丙烯的新型分子,且其稳定性已被理论研究所证实。较为有趣的是,2BB衍生物的LUMO轨道与乙烯和环丙烯的LUMO轨道相似,我们预测2BB衍生物也可发生Diels-Alder反应从而产生一类新型分子。因此该论文首先研究了 2BB衍生物的结构,稳定性,芳香性和化学键,并进一步研究了其与丁二烯以及5-取代环戊二烯的Diels-Alder反应,并对影响Diels-Alder反应的因素作了探究。研究内容包括以下三部分:1.本文采用B3LYP-D3/6-31 1G**方法,研究了 24种2BB衍生物的结构,稳定性,芳香性和化学键。结果显示2BB衍生物中包含一个C=C键,其键长在1.465～1.497A范围内,键级约为1.33,表明该C=C键介于单键和双键之间。基于从头算价键理论方法的计算和分析结果,我们发现这三个衍生物中C=C键包含一个电荷移动化学键和共轭π键,而且该类分子的离子共振结构具有卡宾特性,因此我们将2BB衍生物称为硼杂环卡(B-Heterocyclic Carbene,BHCs)。另外通过计算24种2BB衍生物中BCC三元环的核独立化学位移值表明该三元环具有较强的芳香性。2.基于硼杂环卡宾与环丙烯以及乙烯LUMO轨道的相似性,本文采用B3LYP-D3/6-311G**方法研究了硼杂环卡宾与丁二烯的Diels-Alder反应。研究结果表明上述环化反应是放热反应,反应机理表明内型(endo)加成比外型(exo)加成更有利,所以预测的产物为7-硼-辛-3-烯[4.1.1]螺桨烷衍生物。内型(endo)圳加成的反应能垒与硼杂环卡宾的超共轭芳香性稳定化焓具有很好的相关性。3.本文基于B3LYP/6-31 1G**方法进一步研究了 8种硼杂环卡宾与5种5-取代环戊二烯的Diels-Alder反应。研究表明40个环加成反应均为放热反应。反应机理研究表明,反应具有专一的endo-exo立体选择性,即倾向于endo加成。分析得出次级轨道效应,空间位阻是影响Diels-Alder反应endo-exo立体选择性的主要因素,另外静电相互作用也会影响上述Diels-Alder反应的endo-exo 体选择性。反应的syn-anti立体选择性由空间位阻以及静电相互作用共同决定。3-取代2-硼杂环[1.1.0]1(3)丁烯以及5-取代环戊二烯的超共轭芳香性稳定化能(ΔHASE)与反应能垒具有一定的相关性,表明超共轭芳香性稳定化能对反应物的反应活性具有一定的影响。因此,次级轨道效应,空间位阻,静电相互作用,芳香性稳定化能(ΔHASE)都是影响Diels-Alder反应的因素。该研究所设计的硼杂环卡宾,具有良好的稳定结构和反应活性,将对有机化学以及新型材料设计等领域具有一定的理论价值。
英文摘要：
Cyclopropene is highly reactive due to its high ring strain,and the Diels-Alder reactions of substituted cyclopropenes have been a long-standing research topic in bioorthogonal and organic chemistry.Our previous studies indicated that substituted 2-borabicyclo(1.1.0)but-1(3)-enes(2BB)are a kind of novel molecules and structurally analogous to cyclopropene.The theoretical analyses of the derivatives of 2BB confirmed the stability and likely existence of 2BB.It is interesting to note that the LUMOs of substituted 2BB molecules are similar to those of ethene and cyclopropene,therefore,it can be anticipated that substituted 2BB potentially provides rich opportunities in the rational design of novel molecules via Diels-Alder reactions.In this paper,we firstly investigated the structure,stability,aromaticity and bonding of substituted 2BB,then we extended the study of 2BB to the Diels-Alder reaction with butadiene and 5-substituted cyclopentadiene and explored the factors affecting Diels-Alder reactions.The research content includes the following three parts:1.The structure,stability,aromaticity,bonding of twenty-four substituted 2BB have been investigated by B3LYP-D3/6-311G**method.Calculated results show that the 2BB derivative contains a C=C bond whose lengths are in the range of 1.465-1.497A,and their WBIs are close to 1.33,indicating the inverted C=C bond are between single and double bond.Yet ab initio valence bond(VB)method computations of three derivatives of 2BB showed that the inverted bonds therein are charge-shifted and contains a conjugated π bond,the ionic resonance structures arising from charge-shifting are of carbene characteristics as well.Thus,the derivatives of 2BB can also be considered as B-heterocyclic carbenes(BHCs).The values of nucleus-independent chemical shifts indicating that there is significant aromatic stability in these three-membered rings.2.Since the LUMOs of the BHCs are similar to those of ethene and cyclopropene,here we investigated the Diels-Alder reactions of the BHCs with butadiene through B3LYP-D3/6-311G**method.The results show that the cyclization reaction is an exothermic reaction.The reaction mechanism indicates that the endo addition is more favorable than the exo addition,so the predicted product is 7-bora-oct-3-ene[4.1.1]propellane.Notably,there is a good correlation between the endo reaction barriers and the hyperconjugative aromatic stabilization enthalpies.3.The Diels-Alder reaction of 8 kinds of BHCs and 5 kinds of 5-substituted cyclopentadiene was futher studied by B3LYP/6-311G**method.The results have shown that 40 cycloaddition reactions are exothermic.The investigation of reaction mechanism has shown that the reaction have a specific endo-exo stereoselectivity,which tends to endo addition.Analysis shows that the secondary orbital interaction(SOI),steric hindrance are main factors that affect the stereoselectivity of the Diels-Alder reaction.In addition,the electrostatic interaction also affects the endo-exo stereoselectivity of the above Diels-Alder reaction.The syn-anti stereoselectivity of the reactions are determined by both steric hindrance and electrostatic interaction.Hyperconjugative aromatic stabilization enthalpies(ΔHASE)of 3-substituted 2-borabicyclo(1.1.0)but-1(3)-ene and 5-substituted cyclopentadiene has a certain correlation with the reaction energy barriers,indicating the hyperconjugative aromatic stabilization have a certain influence on the reactivity of the reactants.Therefore,SOI,steric hindrance,electrostatic interactions,and hyperconjugative aromatic stabilization energy(ΔHASE)are all factors that affect the Diels-Alder reaction.The BHCs have novel structures and well reactivity,it will be of great significance to field such as organic chemistry,and new materials.
